X-RAY PHYSICS
Grid cutoff
decrease # of photons reaching the film b/c of misalignment of grid resulting in decrease of density (INCREASED NOISE)
mA
milliampere, this is a current measurement which means that it is related to the QUANTITY of electrons moving from cathode to anode See below picture for important facts about mA Note: mA is a direct relationship with the number of electrons moving, i.e., an electron stream with @ mA of 100 has 2X the electrons of an electron stream with mA = 50 Note: the mA changes the QUANTITY of an electron beam, but does NOT change the quality
Detector systems (fluoro above table vs under table systems
ABOVE table system will give HIGHER lens dose to operator and tech ABOVE table systems actually get the RECEPTOR and the BLADDER closer together and therefore there is LESS blur (think about this type of system which GU radiologists)
Spatial resolution of various X ray techniques
Plain film breast = 15 lp/mm Digital mam = 7 lp/mm Digital radiography = 3lp/mm
Practical applications of the anode heel effect
Please see image
Quality and quantity summary
SEE IMAGE
MQSA breast specific requirements
SEe image
Grid
You use a grid to increase spatial resolution HOWEVER ultimately the grid INCREASES the dose SEe image
Organization of an atom
Z = atomic number = the number of protons A = mass number = total number of protons + neutrons
Air kerma
- This is the TOTAL energy deposited from the ionizating radiation, i.e., the PEAK skin dose based on the potential transfer of energy - Remember, the photons create kinetic energy in the electrons but they also generate heat. BOTH are considered in the KERMA - Another way to say what KERMA is, is an estimation of how many photons are in a unit of air prior to energy striking the skin
Image Optimization (Contrast)
- primary enemy of contrast is SCATTER (you want to reduce scatter) - Note: If you are imaging something with high intrinsic contrast you can increase the kVp more without losing contrast because you don't need to accentuate subtle contrast differences - Note: remember, if you are ****ing with the kVp but wish to maintain a specific image density, you're going to need to adjust the mAs. - Note: You can use a grid to increase contrast. Remember, grids are used for thicker body parts (not babies and feet) because thicker body parts have higher scatter. Also grids will be used when you are using a HIGH kVp given the increased scatter. Also remember that grids INCREASE dose. -S/P Ratio: this is the ratio of secondary xrays hitting the detector vs. primary xrays hitting the detector. LOW s/p ratios are better. - S/P Ratio determinates: 1. Thickness of the body part 2. AREA of the beam. BOTH of the above are going to INCREASE scatter and increase the S/P ratio (bad). Both of these essentially have to do with the fact that you are going to have MORE interactions. Other ways of reducing scatter/increasing contrast: 1. Collimate: reduces the AREA of the beam 2. Compress part: this REDUCES THICKNESS 3. Lower kVp: Reduces number of compton interactions 4. Grid/Air gap: reduce the number of scattered photons.
Unsharpness of images (fluoro)
1. Geometric (like we've talking about) 2. Patient (varying thickness of the patient - typically with DECREASED attenuation at the edges of an image which produces INDISTINCT edges) 3. Receptor (limitations in thes ystem to record exact locations) —Receptor unsharpness is the ULTIMATE limit on resolution —Resolution in a digital system is limited by the SIZE of the pixels Relationship between intrinsic RECEPTOR resolution and FOCAL spot size (R/F) will determine if geometric magnification will increase or decrease the spatial resolution —R/F > 0.5 then magnification will REDUCE unsharpness i.e. INCREASE spatial resolution —R/F < 0.5 then magnification will INCREASE unsharpness i.e. DECREASE spatial resolution Practically speaking, the way we can MAG in breast (this is geometric mag) and INCREASE resolution is the fact that the focal spot is SUPER SMALL (0.3 mm in breast vs 1 mm in other forms of imaging)
Fluoro (general differences compared to regular radiography)
1. Longer exposure times 2. Small focal spots 3. Decreased mA 4. Similar kvP See image
Maximum skin entrance dose for STANDARD fluoro
88 mGy/min or 10 R/min
ionizing radiation
A 15eV photon is required to removed an electron from an atom Remember SMALL wavelength = high energy Hard xrays: these are below .1-.2 nm - these are xrays that we can use for imaging purposes, larger xrays do not generate good image
Breast Compression
Allows you to DECREASE dose and also REDUCES scatter (increased uniformity and decreased thickness) SEE IMAGE for full list of important reasons
Alpha particles
Alpha male of particles, 2 protons and 2 neutron w/ total charge of +2 These guys are highly ionizing because they are very good at pulling off electrons Remember: these do not travel far and cant penetrate far These are not used for medical imaging
Mammo Basics (Accentuating DIFFERENCES between this and normal radiography)
Attenuation between breast tissue (normal) and cancer (abnormal) is very small You HAVE to use LOW KvP to accentuate these differences (typically 25-30 kVp) ANODE (target) is NOT tungsten, instead it is either MOLYbdenum or RHODIUM...So why is this: remember characteristic xrays are formed for different substances, for moly and rho the characteristic xrays are around keV of 16-23 VERSUS 69.5 keV for conventional How do we get the beam to be extremely uniform? - FILTERS: use a moly filter (k-edge 20 keV) therefore everything BELOW 15 or ABOVE 20 gets filtered SEE image for HIGH YIELD facts about breast imaging basics MQSA: does NOT have requirements for specific line pair requirements for digital
Image Noise
Background variation (this is always there) and is contributing nothing useful to the image This is related to the NUMBER OF PHOTON hitting the imaging receptor (The MORE photons that hit the background receptor the LESS obvious the background noise will be and you will INCREASE the signal to noise ratio). not all noise is quantum scatter, some of the noise is related to SCATTER which is caused by 3 main factors: KvP: increasing kVp increases scatter Field of view: The SMALLER the FOV, the LESS scatter (this is because less tissue is being irradiated to create interactions). Also, the smaller FOV i.e. smaller target for the scatter to land on. Thickness: The thicker the skin the MORE interactions occur and therefore more scatter (predominately via comptom interactions)
Quantum Mottle
Basically the concept here is that any xray is NOT COMPLETELY uniform and therefore you're going to get this quantum mottle noise Quantum mottle is MORE NOTICEABLE at low xray beam dose When you INCREASE field size, you DECREASE quantum mottle (as the xray beam uniformity increases (maybe?)) however the amount of scatter and dose BOTH INCREASE
Pulsed fluoro vs regular fluoro
Basically, regular has a constant low mA (higher s) Pulsed fluoro has short bursts of high mA IF the frame rate is 30/s then there is NO DIFFERENCE in dose Benefits of pulse fluoro: - Good for moving patients because it gives less motion blur - Pulse fluoro CAN reduce dose when the pulse rate is LOW than 30/s. 50% reduction in pulse rate = 30% reduction in dose
Window Width
Changing the window WIDTH is what changes the CONTRAST of an image
X-Ray Production
Current is passed through tungsten (reason tungsten is because it has an extremely high melting point) This make electrons extremely energetic and then some "boil off" which is called THERMIONIC EMISSION Note the tungsten filament is a CATHODE (negative charge) which repels the electrons toward an ANODE (positively charged) As they accelerate, they gain kinetic energy KeV, and when they hit the anode they lose energy via three means: 1. Excitation: only makes HEAT, NO x-ray 2. Ionization: xray producing 3. Radiative loss: xray producing Electron cup: This thing sits by the cathode and makes sure the electron beam strikes the target in an acceptable size Anode fact: the smaller the anode, the higher the spatial resolution however you also generate more heat
Dose area product relation to distance (IR fluoro)
DAP is INDEPENDENT on distance Although as you get further away the intensity of the beam drops however the AREA of the beam cross section INCREASES REMEMBER: magnification will INCREASE your Air Kerma but WILL NOT increase the DAP (dose area product)
Indirect vs direct flat panel detectors
DIRECT flat panel detectors use photostimulatable materials such as SELENIUM Indirect flat panel detectors use phosphors to convert photon energy to light energy
Detective Quantum Efficiency
DQE Essentially an estimate of the required exposure level that will be needed to create an optimal image This is a measure of EFFICIENCY Better the DQE, lower the dose needed DQE decreases as the demand for spatial resolution increases DR systems have a higher DQE because they have a near perfect fill factor
Artifacts (I.I. Vs FPD)
FPDs do NOT have pincushion, S distortion, vignette, or saturation artifacts. GHOSTING artifact occurs with BOTH
Window Level
For digital radiography, changing the window LEVEL (or center) will change the BRIGHTNESS of the image Remember, if you are viewing something on a monitor, then the brightness refers to luminance (light emission)
Spatial resolution vs magnification (Fluoro)
For electronic magnification (zoom): cutting the input field in half HOWEVER the OUTPUT field always remains the same size therefore the object has undergone 2X magnification and therefore 2X increase in spatial resolution Note: 1/2 input field = 1/4 of the image brightness therefore ABC kicks in and increases dose THEREFORE input radiation has to be increased 4X
X ray vs Gamma Ray
Gamma ray come from the nucleus of an atom that is trying to become more stable X ray originate from interactions between fast moving electrons and atoms
Geometric vs electronic magnification
Geometric magnification is where you magnify by DECREASING the distance between the focal spot (source of xrays) and the object. Consequence: 1. INCREASED dose which follows a SQUARED relationship 2. Geometric blur is going to increase Note: - You get MORE magnification with a smaller focal spot AND this decreases the degree of blur (SEE IMAGE for alternative explanation of mag for geometric) Electronic magnification: Process of magnification through the use of focusing electrodes. Consequence: 1. If you decrease the FOV, you are decreasing the amount of the phosphor input that is getting irradiated which in turn drops the brightness. In response, the automatic brightness control (ABC) increases the dose! **NOTE** - BOTH forms of magnification are going to INCREASE the dose however the geometric form increases dose MORE A few important notes: 1. Spatial resolution is INCREASED by electronic magnification 2. The main limiter of spatial resolution is the quality of the DISPLAY or TV
Digital Artifacts
Ghosting: This is created from residual image form a prior exposure Pixels gone bad artifact: This can manifest as a square or dark streak
Dose & compensation for geometric magnification
How to compensate for the blur caused by geometric magnification? 1. You can collimate which effectively changes to a smaller INPUT surface and therefore you get MAG and probs less blur from decreased area of irradiation What is best position for II and xray tube: - Want I.I. CLOSE to the patient and you want the x-ray tube FAR away 3 things II close to patient does: 1. Decreases the patient dose (the ABC does not have to ramp dose as much because you have more incident xrays on the detector) 2. Decreases scatter to operator (basically this because the ABC is not having to increase the mA, i.e., there are just fewer Xrays) 3. Increases sharpness/spatial resolution (DECREASE magnification and blur)
Frame averaging (recursive filtration)
Image process technique that adds several images together Purpose: REDUCES quantum mottle and INCREASES S/N Down side: INCREASES motion artifact and ghosting
Direct System:
In this process you can skip the light conversion step The compound that is used it Amorphous Selenium (photoconductor) Remember, both DIRECT and INDIRECT systems use a Thin-Film-Transistor (TFT) to read out the array. There is NO LATERAL DISPERSIONs in a direct system as there is NO light generation (directly go from xray to charge)
Rule of 15%
Increasing the kVp by 15% will DOUBLE the intensity of the xray beam (remember you generate HIGHER AVERAGE ENERGY PER PHOTON and you INCREASE the NUMBER of xrays products via (brems) Increased the mAs by 100% will DOUBLE the intensity as it only has an effect on the number (in al linear fashion)
XRAY physics terms
KvP: think about this as water pressure pushing water out of a pipe but for electrons. The KvP actually represents the PEAK kilovoltage. This system of all electrons moving is non-uniform so setting to 100 KvP basically means the fastest electron is moving at 100 KvP. KeV: This describes the energy of a SINGLE electron within an electron beam Max energy xray = max KvP. You CANNOT have an xray with a higher keV than your KvP. Lowest energy xray is going to be determined by what filter you use. KvP manipulation trivia: 1. Below 69.5 kVp = zero k shell charactersitic x-rays 2. Between 80-150 kVp = 10-25% K shell characterstic xrays 3. Between 150-300 = progressive decrease in contribution Above 300 = negligible contribution Larger Z = more overall xrays produced via bremsstraglung. This is a change in QUANTITY Larger Z = difference characteristic energy peaks (NO change in the number produced via ionization) this is a change in QUALITY
Spatial Resolution (fluoro systems)
Limiter on I.I. System: TV display resolution which is impacted by Raster (scan) lines, bandwidth, and FOV **does display limit of an FDP system?** NOPE. Display with the same matrix as image receptor is normally chose. I.I. Systems have a better pure resolutiona nd change with the FOV
Magnification
Magnification views are important for breast How do we accomplish this? 1. Increase the detector to object distance (this DOES increase the dose) 2. The AIR GAP, this is equivalent to a grid (there is NO human created grid used for mag views) So how do we keep the spatial resolution HIGH with these other changes - DECREASE the size of the focal spot - With smaller focal spot you after to DECREASE the mA so you dont melt your target but you also have to use LONGER exposure times in order to the get enough xrays for a good exposure
Unsharpness (i.e. lack of spatial resolution)
Motion unsharpness: loss of spatial resolution that is attributed to *patient motion* One way that you can fix this is to decrease patient motion OR to decrease the exposure time System unsharpness: This is unsharpness that is related to the DETECTOR. There are a few ways
Focal spot (BREAST)
Must use a SMALLER focal spot as you need HIGHER resolution Consequence is that this INCREASES HEAT Therefore, must you lower mA BUT exposure time must be longer
Major difference between Image intesified system (I.I.) VS. Flat panel detector (FPD)
NO T.V. Required for the flat panel detector Effectively, the difference between the two systems boils down to NOT having to convert the electrical signal back to LIGHT in order to interpret —The output phosphor has to take electrons and then turn them into light that the human eye can see
Distance effect on noise
Noise will increase as the distance between the detector and the source increase by the INVERSE SQUARE LAW
Magnification
Note: magnification increased when the SOURCE if closer to the patient Note: magnification increases when object (patient) is far away from the detector
PPV (Breast)
PPV1 = PPV for SCREENING EXAMS. PPV benchmark is 4.4% Another way of concepualizing (PPV for BR-0, BR-3, BR-4, BR-5) Notice this is EXCLUDING BR-1 and BR-2 which are things we called BENIGN. Importance is that a SCREENING ONLY facility collects PPV data in this category alone. PPV2 = PPV when BIOPSY was recommended, i.e., PPV for BR-4 and BR-5. PPV benchmark = 25.4%. PPV3 = PPV of the biopsied specimens themselves. PPV benchmark = 31% —Other names for PPV3 can be PBR (positive biopsy rate) OR Biopsy yield of malignancy
Focal Spot
Please see image below for testable trivia on the focal spot Remember, the focal spot is where the electrons hit the anode Other important facts about focal spot: 1. Smaller the target angle = smaller focal spot = better spatial resolution 2. Smaller the angle = increased heel effect Notes: 1. Higher the mA, the WIDER the focal spot which means there is blooming 2. Higher the kVp, the smaller the focal spot, thinning
Anode heel effect
Please see the image below for explanation
Vertical resoution
Raster lines x keel factor / 2 X POV (mm)
High level control (air kerma limit)
Remember it is normally 87 mGy/minute or 10 roentgen/min For VERY fat people you can use the HLC which allows you to crank to 20 roentgens/min which = 176 mGy/min NOTE: if you use high level mode, you need audible or visual alarms in addition to the normal time alarm used for traditional fluoro
Dose effect on noise
Remember, the arch nemesis of quantum mottle is MORE PHOTONS If you want to decrease quantum mottle (which would in turn decrease noise), then you can INCREASE the mAs (this is much better than increasing the kVp) Although increasing the kVp would probs decrease quantum noise, it would DECREASE contrast and would probably lead to more compton scatter (AGAIN remember when we are talking about the KeV of a photon, we are referring to the AVERAGE energy) See the below image for a gamesmanship summary
Modulator transfer function (MTF)
This is information recorded/information available This number can never be greater than 1 and is almost always going to be LESS than 1
Last image hold VS spot image (fluoro)
See image
Regulatory dose limits (Fluoro)
See image
Heel Effect (BREAST)
See image: Important to note that the CATHOD side goes towards the chest which is the more intense beam See image
Electronic magnification (in depth)
See images for deep dive Big time points: 1. When you make you FOV smaller (rememebr this is not columnation) you minify LESS which MAGNIFIES the image —Dose is going to INCREASE —Minifying something less means minification LOSS which means the image will be LESS bright unless the ABC cranks up the mAs 2. Electronic mag INCREASES the air kerma (and therefore the skin dose) Why? You increased the number of photons and you focused it over a smaller spot 3. Electronic mag DOES NOT increase the KAP. This is because even though you have increased the juice you DECREASED the cross sectional area of the beam therefore they NULLIFY.
Image Intensifier systems (old school)
See images for in depth discussion and reference Brightness Gain: this refers to the brightness gain that you have from the OUTPUT phosphor relative to what you actually have entering the INPUT phosphor —Brightness gain is related to ELECTRODE AMPLIFICATION (FLUX GAIN) and MINIFICATION GAIN...it actually is as follows brightness gain = (flux gain) x (minification gain) Conversion gain = measure of efficiency of an II which basically = brightness gain —NOTE: the older the system, the LESS brightness gain the system can do. Consequence = MORE dose is needed to get the same output brightness as the machine ages. How to fix the degradation of brightness gain over time: 1. Use a LARGE aperture. Consequence = increases image noise 2. Let automatic brightness control increase dose. Consequence = increased dose 3. Buy a new system General rule of thumb is once the conversion falls >/= 50% then it's time to replace
Bad pixel (FPD)
Shown as a black or white dot. How to improve? Use INTERPOLATION (fills data)
Digital detector types
Storage phosphor (CR) - type of indirect Flat panel detectors (DR) Direct Indirect Indirect (scintillators) = xrays -> light -> charge Direct (photoconductors) = xrays -> charge
Spatial resolution (basics)
This is how close 2 lines can be to each other and still be visible as two distinct objects The term you will see thrown around is LINE PAIRS PER MM Spatial resolution and spatial frequency are essentially the same thing for all practical purposes
Linear Attenuation
The fraction of photons interacting per unit thickness of an absorber,i.e., the fraction of photons removed from the xray at a certain distance Factors in compton scatter, photoelectric effects, and coherent scatter Example: linear attenuation between water, ICE, and vapor (all h20) is DIFFERENT as the density of these is different (remember absorption is related to DENSITY) See image below Mass attenuation: Don't really understand this but basically know that the MASS attenuation between water, ice, and vapor is the same
Image Optimization (Density)
The key to changing the image density is the mAs You can use the 30% rule which means that you have to increase the mAs by at least 30% to get a noticeable change in image density NOTE: Increasing the kVP WILL increase the image density but it also increases the amount of compton scatter. If you are increasing the kVp you are going to get a much larger bang for your buck of density at LOWER energy levels Note: If you want to adjust the kVp but keep the same image density, use the 15% rule. Note: for approximately every 4 cm of thickness on a patient, the mAs needs to be doubled to keep the same density
Half value layer
The layer of material that can reduce radiation levels to half their level The higher the average photon energy, the larger the HVL will be Withe each HVL the average photon energy goes up HVL of an xray does NOT depend on the mAs, remember HVL is a QUALITY measurement HVL does depend on beam filtration HVL DOES depend on anode material HVL DOES depend on kVp Please look at image for great ways of this being tested
XRAY quality
The overall energy of the beam Changes in average energy reflect changes in quality
Isotopes
These are atoms of the same element that will always have the SAME Z (atomic number) but differ in the number of neutrons i.e., they have different A (mass numbers)
Beta particles
These are equivalent to electrons that originate from the nucleus
Characteristic X RAY
These are given off when an electron of a GIVEN element and a GIVEN shell moves to a lower shell, i.e., has to LOSE energy
Filters
These are going to filter the lower energy xrays thus increasing the overall average photon energy which will also increase the HVL
Relationship between half value layer (HVL) and linear attenuation
These are inversely proportional, it's intuitive See image
XRAY Grid
This blocks all of the XRAYS that are NOT traveling within a straight line (which ultimately give you scatter/noise) This is almost equivalent to a collimator in nuclear radiology imaging Grid ratio = density of the grid. As grid ratio goes UP (density goes up), there is LESS scatter and BETTER contrast HOWEVER, the delivered dose will INCREASE with a grid Because you are having fewer photons contact the receptor, the brightness control (which is related to the mAs) INCREASES to compensate Important points: When you use a grid vs. no grid, you INCREASE dose When you use a grid with a higher grid ratio, you INCREASE dose
Bucky grid
This is a grid that moves back and forth very fast so you don't get grid lines (if it stops moving back and forth then you will see grid lines) Bucky factor = the mAs required w/ grid / mAs requirement without grid
Lateral dispersion of light
This is a process by which you lose spatial resolution Basically as with anything in radiology, you want all of your signals to be SPATIALLY in line with where they originated from Unfortunately when light is generated from a CsI (Cesium Iodide) scintillator, it fans out slightly before hitting the PHOTODIODE Note: this is something that happens with INDIRECT systems for capturing images Note: This gets WORSE as the thickness of the crystal increases
Automatic Exposure Control (AEC)
This is essentially an ionization chamber that acts as a timer to determine appropriate exposure based on body part This ionization chamber is placed between the patient an the image receptor AEC only controls the quantity of radiation, i.e., this is a mAs thing Exposure is determined by the density and thickness of the area of the patient placed over the ionization chamber
Predominate form of X-ray tissue interaction in breast mammography
This is going to be PHOTOELECTRIC EFFECTS The avg tube current in breast radiology is approximately 25 KeV which means the beam energy is closer to 10-15 KeV In tissue, energy levels at 25 KeV are going ot have EQUAL photoelectric and comption Below this level, photoelectric will predomiante Above this level compton will predominate
Automatic brightness control (ABC)
This is something that is trying to OPTIMIZE the EXPOSURE In fluoro, think of this is looking at the amount of light from the OUTPUT phosphor ABC limits: - The max number that the ABC can crank to is 87 mGy/min (10 R/min)
Look up table
This is specific to digital radiography and is the major determinate of IMAGE CONTRAST for DR Digital systems have much wider dynamic ranges when compared to plain film which means kVp is less of a determinate You can use higher kVp's with DR because of the LUTs
K-Edge
This is the KeV that corresponds closely to the inner (K shell) binding energy for a given element. Why is this important: you should select a kVp that is going to give you an xray spectrum that is NEAR the K-edge of a contrast agent like iodine or barium Practical application: You should have the kVp up to AT LEAST twice the k-edge of whatever contrast material you're using. EXAMPLE: - We use a kVp of 60-90 when we image with barium because the K-edge of barium is 33. REMEMBER: kVp = the maximum energy of the photon beam (but VERY FEW xrays are actually at the maximum energy) The MAJORITY of the stream is going to be at 1/2 to 1/3 of the kVp putting the average beam energy right at the K-edge for our contrast agent.
Kerma-Area Product (KAP)
This is the amount of Kerma (potential dose) X cross sectional area of the x-ray beam Think of this more as the TOTAL RADIATION used in the exam rather than the ACTUAL dose to the patient
Binding force
This is the attraction between the proton and the electron Always think of binding force having a NEGATIVE potential energy, i.e., energy needs to be ADDED to the system in order for an electron to get ejected Interesting concept: so the CLOSER an electron is to the nucleus, the GREATER the negative potential (more stable electron) The FURTHER away from the nucleus an electron is the LOWER the negative potential and the more energetic the electron (less stable electron) Also good to know that binding energy varies with the INDIVIDUAL ELEMENT and the LEVEL of the electron
Differential absorption
This is the difference between what is absorbed by the body (PE effects) versus what passes right through the body. Word "differential" refers to the fact that different organ substances absorb differently IMPORTANT: 1. Anything that increases photoelectric effects is going to INCREASE absorption. Be examples are density (this is how close the ATOMS are to each other), kVp, K-edge, increasing the Z (atomic number)
Auger electrons
This is the process by which in ionization the ejected INNER SHELL electron hits an outer shell electron and the outer shell electron is ejected. There is NO xray production with auger electrons. As the Z goes up, there is a higher likelihood of producing charactersitics xrays As Z goes down, higher prob of producing auger electrons
XRAY Collimation
This is the process by which you restrict an xray beam physically Increasing collimation DECREASES field size and therefore DECREASES scatter All things being considered, collimation DECREASES NOISE
Binning
This is the process of combining multiple detector elements to make ONE detector element —This process INCREASES signal/noise ratio by decreasing hte amount of QUANTUM MOTTLE —Binning is MOST useful when you are using LARGE F.O.V. Because there are SO MANY pixels that you're going to get a shit ton of quantum mottle —Importance: binning allows you to DECREASE radiation dose
Fill factor
This is the proportion of the detector that is sensitive to xrays in relation to the entire detector area The higher the fill factor, the more efficient the detector Direct Systems: These have nearly a 100% fill factor Indirect systems: do not have a perfect fill factor
Dynamic range
This is the range of exposure intensities that that can be accurately detected by a film system Digital xray has LINEAR response to exposure with LARGER dynamic range Film xray has CURVILINEAR response to exposure with SMALLER dynamic range
Copper filtration
This is used in interventional radiology to reduce risk of skin burns and in pediatric radiology to reduce dose
Pixels
This is what determines your spatial resolution Each pixel can be thought of as a recorded numerical value which represents the BRIGHTNESS level of the display Pixels are in a matrix and the matrix size is VARIABLE (the more pixels in a matrix (assuming stable matrix size) means that the pixel size DECREASES Pixels/unit area is referred to as PIXEL DENSITY. The higher the pixel density the BETTER the spatial resolution Pixel Pitch: This is the distance from the center of one pixel to the adjacent pixel (more easily understood as pixel spacing) 1. Decreased pixel pitch gives HIGHER spatial resolution It is important to note that the spatial resolution of film may still be better because you do not have to rely on pixels
Photoelectric Interactions
This is where the incoming xray hits an INNER electron and causes ejection with the downstram cascade of characteristic xray production when outershell electrons fill the gap. This typically happens at LOWER ENERGY spectrums. The other possibility is that an AUGER electron is created which still can cause damage to tissue. Photoelectric interactions are ALL OR NOTHING, i.e., the energy of the incoming xray has to be GREATER THAN OR EQUAL TO the inner shell binding energy. The probability of a PE interaction is HIGHEST near the inner shell binding energy and DECREASES as you go over that (essentially super high powered electrons just blast through your material) The function is INVERSE to the 3rd power. Probability PE = 1/E^3 . REMEMBER E = incident photon energy The HIGHER the Z, the more likely you will have a photoelectric effect, Prob PE = Z^3 (direct relationship). The nice thing about this cubed relationship is it allows for good contrast differention between things that have SIMILAR Z. PE interactions DO CONTRIBUTE TO CONTRAST WITHIN AN IMAGE (this is a good thing) The bad: 1. Photoelectric interactions generate the MOST dose to patient because they are FULLY absorbed (all or nothing). Remember, even with compton scatter, NOT ALL of the energy is absorbed.
Compton Scatter
This is where the photon enters the human, hits an OUTER (this is important) shell electron, and has enough energy to EJECT the electron. The photon loses energy in the process and changes direction. 1. This happens at HIGHER energies (dominates when you are ABOVE 30 KeV) 2. This DOES ionize via the newly created "compton electron" 3. It deflects the incoming xray (scattered photon) 4. VARIABLE energies can cause compton scatter (in contrast to the all or nothing photoelectric interaction) COMPTON contributes to dose AND it degrades imaging quality as the ejected electron may now go hit the detector. Other facts: 1. Predominant interaction of xrays and soft tissue within the diagnostic imaging range 100 KeV and up 2. DOMINANT interaction that contributes to fog/scatter 3. MAJOR source of occupational exposure 4. Probability of compton interactions does NOT depend on Z, but is dependent on the DENSITY of the material (apparently this is not related to the Z?)
Classical Xray interaction with human
This is where the photon enters the person, interacts with an orbital electron WITHOUT ejection, is deflected, and therefore changes direction (basically this is a lil bitch photon that does not have the energy to knock off an electron) Does NOT result in ionization Does NOT result in net transfer of energy Does NOT contribute to image DOES add small dose to the patient Seen at LOW energy (approx 10 KeV)
Ionization
This is where you actually have EJECTION of an electron Characteristic Xray: These are produced from the process of ionization. They are considered characteristic because they are related to the BINDING ENERGY of the shell involved which is characterstic for different targets
Glass enclosure around electron tubing
This keeps the entire process of electrons moving (cathode and anode) in a vacuum What does the vacuum do? Allows the AMOUNT and SPEED of electrons to be controlled INDEPENDENTLY
XRAY quantity
Total number of xrays or area under a curve in a spectrum diagram
Single phase vs triple phase generators
Trip phase = less ripple with MORE quality and quantity See image
Computed Radiography (CR)
Use Storage Phosphors - this is a substance that can emit light when exposed to a radiation source CR systems can work with a conventional xray machine The storage phosphor holds a latent xray image, it is then exposed to a red laser which liberates the trapped electrons converting the energy to blue-green light. The light is directed onto a photodetector which amplifies the light signal. Sampling Pitch: This is the distance between laser positions as it is reading the plate Important facts: 1. Amount of light detected is proportional to the intensity of the incident x-ray 2. Photostimulated phosphor has wide detectable range 3. The plate is reset by forceable exposure to bright white light. If you do not do this you get ghosting artifact
Geometric unsharpness
Use the image below in order to figure out all questions related to this
Digital Radiography (DR)
Uses a flat panel detector Remember these systems can be DIRECT or INDIRECT
beam intensity/characteristics
combination of mA, kVp, area exposed, time of exposure Technical description is the NUMBER of xrays MULTIPLIED by the energy of the xrays Important notes: 1. the AVERAGE energy of the xray spectrum determines the CONTRAST (must be a change in KvP) because remember we are trying to get the beam characteristics to be close to the K-edge 2. Visualization of LOW CONTRAST objects/tissue can be ENHANCED with increasing the mAs (essentially the equivalent of giving you MORE SIGNAL in MRI terms). Remember, in this case, if we have optimized are KvP, then we cant change the KvP or we are going to ruin the subtle contrast difference that we have achieved. Important examples: 1. Lets say you want to LOWER SKIN DOSE but keep CONSTANT exposure: Increase the KvP by 15% (this means there are going to be fewer photoelectric effects and more of the electrons are going to blast straight through the body and never interact with tissue) THEN 1/2 the mAs.
bit depth
number of bits used to represent each pixel in the resolution Practically speaking, this controls the numbers of shades of gray that can be displayed on a computer monitor To figure out how many shades of gray can be displayed, do 2^(bit depth) The MORE shades of gray that can be displayed, the BETTER the contrast resolution (think about it: you can determine that more things are different from one another)
Bremsstrahlung (radiative loss)
this is how about 80% of xrays are produced. Electrons that come close to the nucleus and then give off energy. The AMOUNT of bremsstraglung interaction is proportional to the ENERGY of the incoming charged particle and the ATOMIC NUMBER of the asorber w/ the higher the Z, the more radiative loss. There does NOT appear to be the actual EJECTION of electrons in radiative loss. Remember you do NOT get peaks from radiative loss, this is a SPECTRUM of energy